The present invention relates generally to integrated circuit manufacturing and more particularly to forming insulated gate field effect transistors.
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. Currently, the gate oxide is formed having a substantially uniform thickness. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
In typical IGFET processing, the source and drain are formed by introducing dopants of second conductivity type (P or N) into a semiconductor substrate of first conductivity type (N or P) using a patterned gate as a mask. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide a gate which provides a mask during formation of the source and drain by ion implantation. Thereafter, a drive-in step is applied to repair crystalline damage and to drive-in and activate the implanted dopant.
There is a desire to reduce the dimensions of the IGFET. The impetus for device dimension reduction comes from several interests. One is the desire to increase the number of individual IGFETs that can be placed onto a single silicon chip or die. More IGFETs on a single chip leads to increased functionality. A second desire is to improve performance, and particularly the speed, of the IGFET transistors. Increased speed allows for a greater number of operations to be performed in less time. IGFETs are used in great quantity in computers where the push to obtain higher operation cycle speeds demands faster IGFET performance.
One method to increase the speed of an IGFET is to reduce the length of the conduction channel underneath the gate and dielectric layer regions. However, as IGFET dimensions are reduced and the supply voltage remains constant (e.g., 3 V), the electric field in the channel near the drain tends to increase. If the electric field becomes strong enough, it can give rise to so-called hot-carrier effects. For instance, hot electrons can overcome the potential energy barrier between the substrate and the gate insulator thereby causing hot carriers to become injected into the gate insulator. Trapped charge in the gate insulator due to injected hot carriers accumulates over time and can lead to a permanent change in the threshold voltage of the device.
As IGFET dimensions are reduced and the supply voltage remains constant (e.g., 3 V), the electric field in the channel near the drain tends to increase. If the electric field becomes strong enough, it can give rise to so-called hot-carrier effects. For instance, hot electrons can overcome the potential energy barrier between the substrate and the gate insulator thereby causing hot carriers to become injected into the gate insulator. Trapped charge in the gate insulator due to injected hot carriers accumulates over time and can lead to a permanent change in the threshold voltage of the device.
Another method to increase the speed of an IGFET is to reduce the thickness of the gate oxide or the dielectric layer at the gate and adjacent the channel. The thinner the gate oxide, the faster the device and the lower the threshold voltage. Current gate oxide thicknesses for production devices are in the 50 angstrom to 100 angstrom range. The technology roadmap projects that as the industry enters the 0.35 micron and 0.18 micron design rule era, gate thicknesses will fall into the 40 angstrom to 20 angstrom range. In this range, maintaining thickness control, pinhole-free small area gates is making the manufacture of these gates difficult. When such gates are made, they are not necessarily reliable and do not last for the normal service life of a device.
There is always a need to increase the performance of devices since there is always a demand for faster and faster computers. Therefore, there is a need for an oxide layer which is much thinner than available today. There is also a need for an oxide layer that is not only thin, but which will also perform reliably over the normal service life of a component incorporating the device. Furthermore, there is need for a device which is manufacturable and which can be made using as few steps as possible. Further there is a need for steps which can be readily controlled during manufacture.
A semiconductor device having gate oxide with a first thickness and a second thickness is formed by initially implanting a portion of the gate area of the semiconductor substrate with nitrogen ions and then forming a gate oxide on the gate area. Preferably the gate oxide is grown by exposing the gate area to an environment of oxygen. A nitrogen implant inhibits the rate of SiO2 growth in an oxygen environment. Therefore, the portion of the gate area with implanted nitrogen atoms will grow or form a layer of gate oxide, such as SiO2, which is thinner than the portion of the gate area less heavily implanted or not implanted with nitrogen atoms. The gate oxide layer could be deposited rather than growing the gate oxide layer. After forming the gate oxide layer, polysilicon is deposited onto the gate oxide. The semiconductor substrate can then be implanted to form doped drain and source regions. Spacers can then be placed over the drain and source regions and adjacent the ends of the sidewalls of the gate.
A method for forming a semiconductor device to produce graded doping in the source region and the drain region includes the steps of implanting the gate material, usually a polysilicon, with a dopant ion that varies the level of oxide formation on the gate. The dopant ion is driven into undoped polysilicon. Nitrogen ions, may also be implanted in the polysilicon to contain the previously implanted ions. For N-type transistors, typically arsenic is implanted. For P-type transistors, typically boron is implanted. Gates are formed. The gate structure is then oxidized. The oxidation process is controlled to grow a desired thickness of silicon dioxide on the gate. The portion of the gate carrying the dopant grows silicon dioxide either more quickly or more slowly. An isotropic etch can then used to remove a portion of the silicon oxide and form a knob on each sidewall of the gate. A heavy ion implant is then done to convert a portion of the lightly doped source region into a heavily doped region within the source region, and to convert a portion of the lightly doped drain region into a heavily doped region within the drain region. Some of the implanted ions are stopped by the knobs on the gate sidewalls. The regions under the knobs do not have as deep an ion implantation resulting in a shallow region beneath the knob. This forms a graded junction having a specific geometry. The geometry of the interface between the lightly doped region and the heavily doped region in the source region and the drain region depends on the geometry (thickness) of silicon dioxide knobs formed on the sidewall of the gate and on the length of the knob.
Advantageously, the dimensions of the silicon dioxide knob can be varied to form a graded channel having a different geometry. The steps are easily performed and one implantation for heavy doping is all that is needed to form the graded junction or doping pattern. The resulting device has a longer life, is more reliable and less likely to fail than devices without graded doped drains and sources. In addition, the geometry of the doping profile can be controlled more precisely using this invention. Information handling systems including such a device are also more reliable and long lived.